1. Field of the Invention
The present invention relates to a capacitor of a semiconductor memory device and a method for manufacturing the same. More particularly, the present invention relates to a ferroelectric capacitor including at least two ferroelectric films having different compositions or different composition ratios and a method for manufacturing the same.
2. Description of the Related Art
Wide use of portable electronic devices, such as mobile information and communication devices, has increased demand for a non-volatile memory that retains data even after the power is turned off. It is anticipated that a ferroelectric random access memory (FRAM) will be a non-volatile memory having benefits of high-speed data processing and low power consumption.
In general, a semiconductor memory device includes a plurality of memory cells, wherein each memory cell includes a transistor and a capacitor. Similarly, each memory cell of a FRAM also includes a transistor and a capacitor. However, a capacitor in a FRAM is a ferroelectric capacitor that uses a ferroelectric substance as a dielectric material.
As shown in FIG. 1, a conventional ferroelectric capacitor is formed by sequentially stacking a first platinum (Pt) film 10 as a lower electrode, a PZT (PbZrXTi1−XO3) film 12 as a ferroelectric film, and a second platinum (Pt) film 14 as an upper electrode.
In the conventional ferroelectric capacitor of FIG. 1, the PZT film 12 is formed having a predetermined thickness using a chemical solution deposition (CSD) or a chemical vapor deposition (CVD).
During formation of this conventional ferroelectric capacitor, defects, such as voids or excess atoms, are formed in an interfacial area of the PZT film 12. Such defects remain in the ferroelectric capacitor even after the PZT film 12 is reheated, which causes the degradation of the FRAM, and in particular, a degradation of the retention characteristics of the FRAM. The retention characteristics and the measurement thereof will be described later.
In an attempt to solve the above problem, a recent study introduced a ferroelectric capacitor in which upper and lower electrodes are replaced with oxide substances, such as IrO2, RuO2, SrRuO3, or the like, to compensate for the leakage of oxygen. In addition, a platinum (Pt) film, which is useful for crystal formation of the PZT film 12, is used as an interlayer to improve the characteristics of the interfacial area of the PZT film 12.
As shown in FIG. 2, such a ferroelectric capacitor includes the PZT film 12, a lower electrode formed under the PZT film 12 by sequentially stacking a first iridium oxide (IrO2) film 16 and a first platinum (Pt) film 10, and an upper electrode formed on the PZT film 12 by sequentially stacking a second platinum (Pt) film 14 and a second iridium oxide (IrO2) film 18.
In this arrangement, the first and second platinum (Pt) films 10 and 14 are used as first and second interlayers, respectively, but are described herein as an element of the upper and lower electrodes, respectively, for convenience of explanation.
The conventional ferroelectric capacitor of FIG. 2 has many advantages over the conventional ferroelectric capacitor of FIG. 1 but still has the above-described defects in the PZT film 12. The defects accumulate in the interfacial area of the PZT film 12 with electrical charges flowing from the upper and lower electrodes for a long retention time. In a capacitor where domains are arranged in one direction by applying a voltage to the capacitor, or in a capacitor heated for an acceleration test, the movement of the electrical charges or the inflow of the electrical charges from the outside exhibits a tendency to increase.
As a result, an electric field is induced in the PZT film 12 in a direction of an external electric field causing the polarization of the ferroelectric domains. The movement and accumulation of the electric charges continues which increases the strength of the electric field induced in the PZT film 12.
If the induced electric field exists in the PZT film 12 when the polarization directions of the ferroelectric domains are switched to the opposite direction by applying a voltage to the PZT film 12 after a predetermined time, new polarization states of the ferroelectric domains become unstable.
As described above, since conventional ferroelectric capacitors include an induced electric field in a PZT film due to defects of the PZT film, new polarization states of ferroelectric domains become unstable when the polarization direction of the ferroelectric domains is switched to the opposite direction by applying a voltage to the PZT film. Therefore, as the size of the ferroelectric capacitor decreases with the improvement of an integration density of the FRAM, the concentration of defects increases while several processes are processed. Accordingly, the polarization states of the domains of the PZT film become more unstable every time data is written. Thus, as the integration density increases, the reliability of the FRAM including such a conventional ferroelectric capacitor is degraded.